An excessive amount of in-rush current may occur through a main switch when a switching regulator is initially driven. The in-rush current shortens a life of a small-capacity power supply such as, e.g., a battery, and is particularly lethal to a limitary coin-cell having a maximum limit current ranging from 30 to 100 mA. Therefore, in order to prevent occurrence of the in-rush current, the switching regulator includes a soft start circuit.
Conventional soft start circuits are largely divided into an analog type and a digital type.
An analog soft start circuit operates in such a manner as to slowly increase a reference voltage and apply the reference voltage to an error amplifier in order to suppress the in-rush current. The analog soft start circuit includes a high capacitance capacitor or a small magnitude of current source in order to suppress the in-rush current, resulting in problems such as a reduction in a degree of circuit integration and an increase in a sample variation of the in-rush current.
The digital type soft start circuit operates in such a manner as to delay an initial driving time of the switching regulator by means of an oscillation clock in order to suppress the in-rush current. However, there are problems that the digital type soft start circuit requires a turn-on procedure of the switching regulator, and the sample variation of the in-rush current becomes higher when the oscillation clock fluctuates.